1. Field of the Invention
This invention relates to an adaptive dynamic range encoding method and device conveniently employed for compressing picture data or the like. More particularly, it relates to a dynamic range encoding method and device which the replay picture quality may be improved when picture are recorded with a high compression density.
There has hitherto been known an adaptive dynamic range encoding device (ADRC) in which picture data supplied on the block basis, in which each block is made up of a pre-set number of pixels, are compressed in data length and outputted. With the dynamic range encoding device, the dynamic range as a local feature of a picture is defined on the block basis and redundancy mainly in the level direction is adaptively removed. The dynamic range encoding device also features reduced error propagation in the spatial direction.
That is, picture data divided into blocks assume approximately equal level values because of strong local correlation. In addition, as shown in FIG. 1, in a 0-255 dynamic range of 8-bit data, one-block dynamic ranges A and B required for re-quantization of the block account for extremely small proportions. Consequently, smaller numbers of bits suffice for re-quantization. Thus the redundancy in the level direction may be significantly lowered by detecting the maximum and minimum values of picture data on the block basis and defining the one-block dynamic range.
Taking an example of compressing 8-bit picture data to 3-bit picture data, the adaptive dynamic range encoding device executes arithmetic and logical operations shown by the equation (1) ##EQU1## where DR, n, L, Q and L' denote the one-block dynamic range, bit allocation, data level of the pixel in the block, code for re-quantization and a restoration value, respectively, to divide a range between the maximum value MAX and the minimum value MIN in the block as shown at (A) in FIG. 1 by way of re-quantization.
It is possible with the adaptive dynamic range encoding device to statistically reduce the variance of errors. It is also possible to increase bit allocation because of the narrow quantization step. Consequently, the device is susceptible to restoration distortion or to the noise applied to the input picture data to only a lesser extent.
There is also known an adaptive dynamic range encoding device in which arithmetic and logical operations indicated by the following equation (2) are carried out to directly output the maximum value MAX and the minimum value MIN in the block as restoration values: ##EQU2##
With the latter type adaptive dynamic range encoding device, since the maximum and minimum values are restored without distortions, the S/N ratio of picture data does not deteriorate despite reiteration of encoding and decoding operations a number of times.
However, the former type conventional adaptive dynamic range encoding device suffers from the drawback that the S/N ratio of the picture data deteriorates each time the encoding and decoding operations are executed.
The following Table 1 illustrates the values of the S/N ratio as obtained on encoding and decoding five standard digital pictures (Y data) supplied by the Television Association, namely a picture of a mountainous village in Switzerland (picture a), a picture of a tulip (picture b), a skin-colored chart (picture c), a woman wearing a hair-band (picture d) and a picture of weather forecasting (picture e), using the former type adaptive dynamic range encoding device, by 3-bit quantization, on the block basis, in which each block consists of 3 lines by 6 pixels for a field.
TABLE 1 ______________________________________ ADRC S/N(db) of S/N(db) of conventional conventional impertinent picture ADRC circuit ADRC circuit ______________________________________ mountainous village in 35.89 35.94 Switzerland (picture a) 25.25 35.94 tulip (picture b) 37.19 37.22 26.61 37.22 skin-colored chart 48.34 48.22 (picture c) 38.15 48.22 woman wearing a hairband 43.38 43.31 (picture d) 32.59 43.31 weather forecast 42.22 42.24 (picture e) 30.41 42.24 ______________________________________
That is, the S/N ratio of the picture a of the mountainous village in Switzerland, which is 35.89 (db) on an encoding/decoding operation, as shown at the upper row for the picture a of Table 1, deteriorate to 25.25 (db) on reiteration of the encoding/decoding operations five times, as shown at the second row of Table 1. Similarly, the S/N ratio of the picture a of the tulip, which is 37.19 (db) on an encoding/decoding operation, as shown at the upper row for the picture b of FIG. 1, deteriorates to 26.61 (db) on reiteration of the encoding/decoding operations five times, as shown at the second row of Table 1.
On the other hand, with the latter type adaptive dynamic range encoding device, the S/N ratio of the picture a of the mountainous village in Switzerland on an encoding/decoding operation is 35.94 (db), while that on reiteration of the encoding/decoding operations five times is also 35.94 (db), so there is no deterioration of the S/N ratio. Similarly, the S/N ratio of the picture a of the tulip on an encoding/decoding operation is 37.22 (db), while that on reiteration of the encoding/decoding operations five times is also 37.22 (db), so that there is no deterioration of the S/N ratio.
However, with the latter type adaptive dynamic range encoding device, the quantization step widths are broader than those of the former type device. Consequently, if, with a block having a broad dynamic range, the picture data which is spatially continuous and substantially on the same level exists in the vicinity of the boundary of the quantization level steps, the picture portions which have appeared as continuous picture portions are divided into discrete portions having widths corresponding to the quantization step widths, thus presenting considerable restoration distortions and visually outstanding defects.
Referring to FIG. 3, showing the level distribution of luminance data (Y data) for a block, the maximum value (MAX) and the minimum value (MIN) of the luminance data in the block are 207 and 42, respectively, with the dynamic range for the block being 165, there being a planar area in the vicinity of the level value of 50. If the luminance data for the planar portion is processed with a multi-stage 3-bit re-quantization with the above-mentioned latter type adaptive dynamic range encoding device, the quantization step width becomes equal to approximately 23.57, with the level distribution of the luminance data (Y data) of the block becoming as shown in FIG. 4, in which the planar portion in the vicinity of the level value of 50 being separated into a portion with the level value of 42 and a portion with the level value of 65 as a result of re-quantization. If the picture data presenting the same level is re-quantized into different level values, a problem occurs in that deterioration near the picture edges becomes noticable.
The problem that exists due to re-quantization of the picture data of the same level to different level values becomes more pronounced for the lower bit rate, that is, for the higher degree of data compression.